A communication system is the generation, transmission, reception and decoding of information, which may be represented as a series of “0”-states and “1”-states; and is extremely important for society. In optical communication systems using directly modulated lasers, frequency chirping may occur. This is an effect causing the optical wavelength (or frequency) to be dependent on the optical power. In other words, the wavelength (or frequency) of a “0”-state will differ from that of the “1”-state. Frequency chirp in communication systems are considered undesirable and far from optimal, as it limits transmission distance due to chromatic dispersion in the transmission fiber which converts the chirp-induced frequency broadening to time-broadening which causes neighboring symbols to overlap and therefore to be erroneously decoded. As a consequence, frequency chirped modulation is not preferred in optical communication systems. Rather, modulators or lasers are made such that frequency chirping is suppressed enough to be used for optical communication systems.
Chirp-free operation requires either chirp-managed lasers or external modulators; both of which come with high price and increased power consumption and heat. On the other hand, modulators or lasers with frequency chirping come with low cost. There is therefore an economical asset if frequency chirped modulators can be tolerated better in for example optical communication systems.
There are solutions, which allow frequency chirped modulators to be used in optical communications systems, but these have means for compensating the frequency chirping. This means that for example filters or more complex hardware solutions are required to be implemented in such a way that the dynamic line width broadening is being eliminated and hence not exploited.
What is missing is a communication system that not only tolerates frequency chirped modulation, but also takes advantage of the dynamic line width broadening inherent in the frequency chirped modulation.
In optical communication systems, there are two well-known detection techniques:                Direct detection        Coherent detection        
Direct detection is the detection of amplitude only whereas coherent detection is the detection of both amplitude and phase. Coherent detection has many advantages over direct detection, including higher sensitivity than direct detection and is therefore increasingly preferred in long-reach (core network) communication systems where transceiver cost is shared by a high number of users opposed to metro and access networks which are very sensitive to transceiver cost. Coherent detection requires, however, information of the carrier phase as the signal is demodulated by a local oscillator (LO) controlled by a phase-locked loop that serves as an absolute phase reference. Operation with phase locked loops puts strict requirements on the system side. Two well-known requirements for phase locked loops are:                Synchronisation between the LO and the encoder        Narrow optical line width of the LO and the encoder        
If these requirements are not met, coherent detection is not working properly. The phase locked loop can be made either in the optical (analog) domain or in the digital domain with digital signal processing (DSP). Regardless of how the phase locked loop is implemented, coherent detection is always required to operate with high cost lasers with narrow optical line widths. In future optical communications systems for metro and access networks, there is a need for a detection technique that provides a low cost solution.